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Solar pond number crunching.

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Posted by SGBotsford on November 9, 2006, 7:59 pm
 
Suppose that I took a reasonably large dugout, put a board rim around
the edge, and using greenhouse hardware, put a plastic layer over it,
and inflated it.  Since  there is no need for people to go in and out,
don't bother with a frame.  I'm trying to figure out if I can run a
greenhouse year round with a solar pond to carry heat from the summer
to the winter.  In my climate a greenhouse uses 250,000 BTU/sqft/year
with double poly covering (r2, 10,000 F degree cooling days).  This
assumes full heat recovery on ventilation (bad assumption.)

I've heard of solar ponds being used as a passive annual heat storage
mechanism.

Suppose that the dugout is 100' long, 30' wide, and 14' deep.  Long
dimension of dugout is E/W


Scenario 1:  Pond was filled in spring with 10 C degree water. Latitude
is 55 degrees N.  Sun shines 80% of the time, 20% clouds.  Assume that
enough heat leaks to melt any snow that falls on it.

Q1.  With a single layer of plastic what is the equilibrium temperature
in February?

Q2   Is it worth it to put a second layer of plastic?

Q3   Given the low angle of the sun in winter, would it be worth while
to put opaque white plastic on the north half, and clear plastic on the
south half?
Discussion: Assume white plastic has reflectivity of 80% Goal: Increase
therm collection in winter, but with less heat collected in summer. At
this lattitude, sun at noon is only about 10-15 degrees above the
horizon for the 12 shortest day weeks of the year.

Q4.  Is it worth while to circulate the water?
Discussion: Temperatures may be high enough that most water critters
won't be happy.  By adding materials such as lime you can cause the
silt to settle, leaving the water fairly clear.  Water heated at the
bottom will convect on it's own.  If, however all the absorption is
occuring in the top few feet, then circulating the water will be worth
while.

Q5.  Would it be worth while to float a layer of bubble wrap on the
surface to retard evaporation?
Discussion: One of the major mechanisms of heat loss will by by water
evaporating from the surface, and condensing on the inside of the
plastic.  Fogging of the plastic could reduce the amount of light at
the surface of the water.

Q6.  Would it be worth while to impede the convective circulation of
the water.
Discussion: This can be done by creating a salinity gradient, but his
takes a huge amount of salt.  Suppose instead you packaged the water in
clear plastic bags.  (Still a lot of work, but bags can't leak into the
watertable.)   (Or a use for millions of plastic pop bottles.)
Since there is effectively no pressure, you can make the bags very
flimsy.  There would be enough space between the bags for some
circulation.

Q7.  If the top of the dugout freezes will it be clear, or white?

Discussion:  If the water got hot during the summer, it won't have a
lot of disolved air in it.  I've seen natural ice on a river that was a
foot thick and clear.  So if there are enough bags on the surface, the
surface may freeze while the bottom is still warm.


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